let b:undefined = undefined
let c:null = null
// let z:string = undefined

function doSomething(x:string|null){
  if(x===null){
    // 做一些事情
  }else{
    console.log('hello'+x.toUpperCase());
    
  }
}

function liveDangerously(x?:number | null){
  console.log(x!.toFixed());
}
